1. Field of the Invention
The present invention relates to a hollow rotor blade of a gas turbine, and more particularly to a lightweight hollow rotor blade having high mechanical strength provided by enhanced fatigue strength of bonded portions between the skin and a core of the blade.
2. Description of the Related Art
A hollow rotor blade of a gas turbine in the related art includes a shaped core accommodated in a hollow space formed between two skins constituting blade surface outer plates. The core is bonded to the insides of the two skins. In examples of such hollow blades, a honeycomb core 3 is inserted in a hollow space between skins 1 as shown in FIG. 8, or a core 4 or 5 shaped by superplastic working is inserted in a hollow space between skins 1 as shown in FIG. 10 and in FIG. 11.
In the hollow rotor blade shown in FIG. 8, the honeycomb core 3 is inserted in a space formed between the two skins 1, and the same honeycomb core 3 and the skins 1 are bonded to each other.
In the hollow rotor blade shown in FIG. 10, a corrugated plate core 4 is inserted in a space formed between two skins 1 having radially inner ends held by dovetails 2, and the corrugated plate core 4 and the skins 1 are bonded to each other.
The hollow rotor blade shown in FIG. 11 was invented by the present inventors and disclosed in Laid-Open Japanese Patent Specification No. 2-125902 (1990). This hollow rotor blade comprises a dimpled core 5.
The bonded portions between the above-described skins and cores are respectively shown in FIG. 9 and in FIGS. 13 to 15. FIG. 9 is an enlarged view of a portion marked D in FIG. 8, and FIGS. 13 to 15 are enlarged views of a portion marked F in FIG. 12. FIG. 12 is a cross-sectional view taken along line E--E in FIG. 10 or in FIG. 11. FIG. 13 shows the case of bonding by brazing, FIG. 14 shows the case of liquid-phase diffusion bonding, and FIG. 15 shows the case of solid-phase diffusion bonding.
In the above-described hollow rotor blades of a gas turbine in the related art, due to the fact that cores shaped by such techniques as superplastic working are directly bonded to skins through brazing, liquid-phase diffusion bonding, solid-phase diffusion bonding or the like, stresses in the skins generated by aerodynamic loads of the moving blades directly act upon the bonded portions of the core. Therefore, cracks are liable to arise at the bonded portions, especially at the portions marked X in FIGS. 9, 13, 14 and 15 which are stress concentrating portions inherent to the bonding configuration of the skins and the core. This degrades the fatigue strength.
Therefore, in the above-described hollow rotor blades of a gas turbine in the related art, a stress level of the skin was lowered and the necessary fatigue strength was obtained by employing rather thick skins.